Field
This disclosure relates generally to an aerodynamic structural element on an aircraft that includes built-in tension to reduce compression loads on the element during flight and, more particularly, to a graphite composite main wing spar having an I-beam shape including upper and lower spar caps and a center web, where the spar caps have a higher coefficient of thermal expansion (CTE) than the web that causes them to be in tension after thermal curing, which reduces compression loading on the wing skin during aircraft flight.
Discussion
A fixed wing aircraft will have a lift-to-drag (L/D) ratio that is defined by the total aerodynamic lift generated by the aircraft divided by the total aerodynamic drag generated by the aircraft as it moves through the air, where the greater the L/D ratio the higher the aerodynamic efficiency of the aircraft. One of the principal techniques of obtaining a higher L/D ratio of the aircraft is to use wings that have a high aspect ratio, i.e., the length of the span of the wing divided by its width, where wings that are very long and narrow tend to produce higher L/D ratios. Unfortunately, high aspect ratio wings suffer from excessive structural loads due to wing bending, where the lift generated by the wing tends to bend the wing upward. This bending causes compression forces to develop in the upper surface of the wing and tensions forces to develop in the lower surface of the wing. The upper surface of the wing is typically considered the critical structural surface of the wing because it develops the compression loads in resonance to wing bending, where compression loads on long thin structural elements can cause these elements to buckle, which often occurs well before the structural element exceeds its compression limit.
Compression induced buckling on the upper surface of a wing is a major aircraft design consideration. Two methods that are typically employed to prevent wing buckling include enhancing the stiffness of the upper surface of the wing by adding reinforcement materials and decrease the aspect ratio of the wing. However, the first solution adds weight to the aircraft and the second solution reduces aircraft performance neither of which is desirable.
An aircraft wing generally needs to have a structural configuration that makes it very strong and stiff, but also allow it to be as light as possible. The structural configuration of an aircraft wing often employs a main wing spar having an I-beam shape that extends the length of the wing, where the spar includes upper and lower spar caps connected by a web. A series of ribs are generally coupled to and extend across the wing along the length of the spar. The wing spar typically carries the loads during flight and the weight of the wings while the aircraft is on the ground, and when the wing bends during lift, most of the compression loads are carried by the upper spar cap. Thus, the main wing spar and the wing skin are built and configured so that their inherent stiffness is high enough to prevent compression buckling as a result of aircraft lift.